1. Field of the Invention
The present invention pertains to the suppression of optical noise in light systems. In particular, the present invention relates to translation devices by which input and output elements in a spatial filtering system are moved synchronously along parallel paths which are perpendicular to the optic axis of the spatial filtering system. This lateral motion of the elements causes the object, the image, and the information-carrying light to move laterally relative to the optical-noise-producing sources in the system, thus averaging out the optical noise pattern and yielding a noise-free image at the output element.
2. Description of the Prior Art
Optical processing systems, particularly using highly coherent light sources, such as lasers or black-bodies, are well known. In a typical arrangement using photographic film for the input and output element, a coherent light beam passes through the input film which bears information to be processed. The light beam, impressed with the information, then passes through a Fourier transform lens in whose back focal plane is located the appropriate spatial filter to carry out the desired operation on the Fourier spectrum. The processed light beam is inverse Fourier transformed by a second lens to produce the "processed" image on the output film. The optical data processing system thus functions as an optical computer by performing mathematical operations on information transported by the light beam.
In addition to the transforming lenses and the spatial filter, other optical elements, such as lenses,
ARE REQUIRED IN THE OPTICAL DATA PROCESSING SYSTEM TO GUIDE AND FOCUS THE LIGHT BEAM. Optical noise arises from the interaction of the light beam with bulk and surface imperfections in all these optical elements. Examples of such noise-producing imperfections are bubbles, inclusions, striae and impurities in the lenses, and dust and other foreign matter on all the surfaces. Lens surfaces are also susceptible to micro-fissures produced as a result of the grinding and polishing operations in manufacturing the lenses. Such minute imperfections are usually of little or no consequence when ordinary light is used. However, when light that is highly coherent interacts with such imperfections, significant amounts of optical noise are generated.
The optical noise arises from the diffraction patterns produced in the light beam when the coherent light is incident on the imperfections. This optical noise manifests itself as a random collection of wavy lines and circles. Bulls-eye type patterns result from interference of the coherent light beam with light reflected at the interfaces of the various lens elements. These designs are registered on the film or other output device superimposed on the desired image. The term "cosmetic quality" is used to describe the extent of freedom from such noise manifestations in the image. Because of the low tolerances for error in coherent light processes such as optical data processing, optical communications, and holography, it is essential that techniques be discovered to nullify the effects of such optical noise.
Various devices and techniques have been suggested to reduce coherent optical noise. In particular, attempts have been made to nullify the noise by averaging the noise pattern at some point in the optical system to a constant background. However, efforts to develop workable and practical noise averaging systems have thus far met with only limited success.
The noise averaging method of Thomas, as described in Applied Optics 10, 517 (1968), uses a tilted optical flat in the laser beam. During the exposure of the output film this flat is rotated about the optic axis of the optical system. This rotation has the effect of rotating the individual noise sources with respect to the laser beam, and hence averaging out their noise patterns in the beam, while the desired output image is itself stationary. However, the rotating optical flat also causes the Fourier spectrum incident on the spatial filter to rotate. Consequently, the spatial filter must also be rotated, and a very accurate feedback system must be added to synchronize the motions.
The technique of Grabowsky et al, as described in Applied Optics 10, 483 (1971), uses a rotating lens, causing the noise pattern to rotate while the desired image remains stationary, thus averaging out the noise. However, to keep the image stationary, the optic axis of the rotating lens must coincide exactly with the axis of rotation. In a multiple-element lens, this condition is virtually impossible to achieve due to the unavoidable misalignment of the optic axes of the individual elements. Furthermore, in multiplelens systems, a separate rotating device is required for each lens, adding to the alignment problems.
In U.S. Pat. No. 3,729,252, Nelson proposes using n separate light sources, each producing a separate noise pattern displaced from all the others in the output plane, while the desired images from all n sources coincide. The effect is to reduce the signal-to-noise ratio by a factor of 1/n. However, the n light sources produce n individual, displaced, and possibly overlapping Fourier transform spectra. Consequently, to carry out the desired processing, a multiple spatial filter must be employed, with the desired filter characteristics centered on each of the n displaced Fourier spectra. Such a multiple spatial filter can be very difficult to fabricate if the desired filter characteristic is complicated.